1. Field of the Invention
The present invention relates to apparatus for a parallel path Coriolis mass flow mass meter and, more particularly, to such a mass flow meter which is easier to fabricate and which has improved measurement accuracy than prior art designs.
2. Description of the Prior Art
The art of mass flow rate measurement teaches that when a fluid flows through a rotating or oscillating conduit Coriolis forces are produced which are perpendicular to both the velocity of the fluid moving through the conduit and the angular velocity of the rotating or oscillating conduit. The magnitude of these Coriolis forces is proportional to the product of the mass flow rate and the angular velocity of the conduit. Meters which make use of this phenomenon are termed Coriolis mass flow rate meters.
In general, Coriolis forces that appear in these mass flow rate meters are rather small. Consequently, sensitive and precise instrumentation was often employed in early Coriolis mass flow meters in order to accurately measure the small Coriolis force effects, such as conduit deflection, which resulted from moderate mass flow rates and reasonable angular velocities. Such instrumentation was usually quite expensive. In addition, the angular velocity of the conduit also had to be accurately measured and controlled in order to determine the mass flow rate of the fluid passing through the conduit as a function of the magnitude of the generated Coriolis forces.
A mechanical configuration and measurement technique which, among other things, avoids the need to measure and control the magnitude of the angular velocity of the conduit and also accurately and sensitively measures the Coriolis force is taught in U.S. Pat. No. Re. 31,450 (issued to Smith on Nov. 29, 1983 and hereinafter referred to as the '450 reissue patent). This patent discloses a mechanical configuration which incorporates a U-shaped flow tube, devoid of pressure sensitive joints, which has its open ends attached to opposite sides of a manifold. When so mounted, this flow tube is capable of being oscillated about an axis perpendicular to the side legs of the U-shaped tube. This axis is located near the tube-manifold interface and is situated in a plane in which the U-shaped tube lies at rest. This plane is hereinafter referred to as the midplane of oscillation. When fluid flows through the mounted U-shaped flow tube, the filled flow tube oscillates. These oscillations are sufficient to cause the free end of the flow tube to pass through the mid-plane of oscillation, and thereby generate a Coriolis force couple which elastically deflects the free end of the flow tube about an axis. This axis is located in the plane of the flow tube midway between and parallel to its side legs. By judicious design of the resonant frequency of the flow tube oscillating about this axis and another axis orthogonal thereto, a mechanical situation is created whereby the forces which oppose the generated Coriolis forces are predominantly linear spring forces. Consequently, through use of such a design, these spring forces cause one of the two side legs of the flow tube to pass through the midplane of oscillation before the other side leg does so. As such, the mass flow rate of the fluid that flows through the flow tube is proportional to the width of the time interval occurring between the passage of the respective side legs of the tube through the mid-plane of oscillation. This time interval and, hence, the mass flow rate of the fluid can be accurately measured using optical sensors as disclosed in the '450 reissue patent, or by using electromagnetic velocity sensors, as disclosed in U.S. Pat. No. 4,422,338 (issued to Smith on Dec. 27, 1983).
The '450 reissue patent also teaches the use of a spring arm which extends from the manifold along with the U-shaped flow tube. When this spring arm is sinusoidally driven in opposition to the U-shaped flow tube, the combination of spring arm and U-shaped flow tube operates as a tuning fork. This operation substantially attenuates undesirable vibrations occurring at the tube-manifold and spring arm-manifold interfaces. This attenuation is extremely advantageous for the following reason. In practice, these undesirable vibrations, often occur, particularly at the tube-manifold interfaces, with sufficient intensity to effectively mask tube movement caused by the small Coriolis forces and thereby introduce significant errors into the time interval measurements of the passage of the side legs of the U-shaped tube through the mid-plane of oscillation. Because the mass flow rate is proportional to the time interval measurements, these errors inject significant inaccuracies into the measured mass flow rate. Tuning fork operation substantially cancels these undesirable vibrations and thereby significantly increases measurement accuracy. In addition, reducing vibrations that occur at the manifold also decreases long term fatigue effects induced by vibrations that might otherwise occur on the meter mounting structure. The substitution of a second flow tube, having a similar configuration to the first flow tube, for the spring arm provides an inherently balanced tuning fork structure. The inherent symmetries in such a structure further reduce undesirable vibrations and thereby further increase measurement accuracy. This teaching has been recognized in the design of densimeters wherein measurements of the resonant frequency of filled flow tubes are used to determine the density of fluids in the tubes. See, for example, U.S. Pat. Nos. 2,635,462 (issued to Poole et. al. during April 1957) and 3,456,491 (issued to Brockhaus during July 1969).
The art also teaches the use of a serial double flow tube configuration in a Coriolis mass flow rate meter. Such a configuration is described in U.S. Pat. Nos. 4,127,028 (issued to Cox et. al. on Nov. 28, 1978); 4,192,184 (also issued to Cox et. al. on Mar. 11, 1980) and 4,311,054 (also issued to Cox et. al. on Jan. 19, 1982). Here, incoming fluid sequentially passes through one flow tube, then through an interconnecting conduit and lastly through another flow tube. Unfortunately, series type double flow tube meters possess an inherent drawback: since all the fluid must pass through two flow tubes instead of one, the fluid pressure drop across the meter is double that of a non-serial type flow meter. The one way to compensate for this doubled pressure drop is to double the pressure at which the incoming fluid is supplied to the meter. Unfortunately, this often entails increasing the pumping capacity of the entire fluidic system that supplies fluid to the meter.
An alternate configuration involving parallel flow tubes is disclosed in U.S. Pat. No. 4,491,025 (issued to Smith on Jan. 1, 1985 and hereinafter referred to as the '025 patent). Here, incoming fluid is evenly divided between and flows into parallel, illustratively two U-shaped, flow tubes rather than sequentially passing through two serially connected flow tubes. At the output end of each parallel flow tube, the fluid is combined in a drain manifold and from there exits the meter. The two flow tubes are then sinusoidally oscillated. As the fluid moves through both flow tubes, Coriolis forces are produced which alternately deflect adjacent legs of the tubes and, in turn, permit time interval measurements to be made in order to determine the mass flow rate of the fluid.
The parallel flow tube design provides significant advantages over the discussed prior art designs that utilize single or serially connected flow tubes. First, each parallel flow tube may be constructed with relatively thin walls which, in turn, provides increased sensitivity. As the wall thickness of a flow tube decreases, the mass and rigidity of the tube also decreases which, in turn, increases tube deflection caused by Coriolis forces. Increasing the deflection for a given mass flow rate advantageously increases the sensitivity of the meter. Second, parallel tube flow meters are, in general, operationally more stable than either single flow tube or serial flow tube meters. This occurs because the fluid flowing through both tubes results in a dynamically balanced pair of tuning fork tines, i.e., as the mass of one tine varies due to increased fluid density so will the mass of the other tine. Third, parallel flow tube meters are less sensitive to error-producing external vibrations and, hence, provide more accurate fluid flow measurements than do single tube or serial tube flow meters. This occurs because the time interval measurement sensors can be mounted on the flow tubes without a physical reference to any structure that is immutably fixed with respect to the mid-planes of oscillation for the tubes. Fourth, parallel flow tube meters exhibit less pressure drop across the entire meter than does a serial flow tube meter.
Unfortunately, difficulties exist with the parallel flow tube meter design. For one, fabrication of these meters is time consuming and hence costly. In addition, at high flow rates cavitation can occur in the fluid as it exits the meter. This, in turn, can cause vibrations that could lead to measurement inaccuracies.
Accordingly, a need exists in the art for a parallel path Coriolis mass flow rate meter which can be readily fabricated and which minimizes the possibility of cavitation.